I can recommend Intellij as well, i tried Netbeans, Eclipse as well, but Intellij was easy to use yet powerful. It also looks really nice in my opinion.
In case you need a small guide on how to debug your plugins with breakpoints and all the fancy stuff, i can recommend my own guide over here. It’s much easier than the way stated in the docs, which requires you to have sponge running as a seperate project.
Also you can use Intellij for everything (limited in Community though) including: Java, PHP, Javascript, Golang, Haxe etc.
Haven’t found a language Intellij doesn’t support. In case you open a file with not yet installed extension, Intellij just shows a small window to install the plugin you need. So you don’t even need to look for a markdown plugin, Intellij offers you to install it when you open a .md file.
Support for Maven,Gradle etc. is out of the box and really good.